Gulf of Maine Warming Update: 2023

Documenting one more year in a decade of above-average regional temperatures

Author

Adam Kemberling

Published

December 29, 2023

Over the past decade, scientists at the Gulf of Maine Research Institute have led a body of research that highlights the rapid pace of warming in the Gulf of Maine. To keep you informed, we share seasonal and annual updates about conditions in the Gulf of Maine.

For analyses like these that characterize environmental conditions for a particular area—in this case, the Gulf of Maine—it is important to be clear about the spatial extent that defines the region of study; different domain boundaries could produce different results. The spatial area we use to define the “Gulf of Maine” is displayed in Figure 1, and has remained consistent throughout our seasonal and annual warming updates.

An overhead view of the Gulf of Maine region. Landmasses and political boundaries for the United States and Canada are displayed. A blue dotted line outlining a box with a transparent blue fill is shown to demark where satellite data for the analysis has been used.

Figure 1: Spatial domain used for Gulf of Maine SST analyses. Dotted line denotes region of study for this analysis. Depth contours are colored at 100m intervals to a maximum of 600m, deeper blues indicate deeper water depths.

Highlights from Another Remarkably Warm Year

  • With an annual average sea surface temperature (SST) of 52.67°F — more than 2.68°F above normal — the Gulf of Maine experienced its 5nd warmest year on record in 2023. This year fell short of the previous warmest year on record — 2021 — 1.42°F, but was record-breaking in a number of regards.

  • In nine of the twelve months this year, average monthly SST was within the top 3 warmest among all years on record.

  • The most extreme temperatures in 2023 occurred in November and December, each setting new records for highest monthly average SST in the Gulf of Maine.

  • The “coolest” monthly average temperature anomalies for 2023 occurred during July and September, but were still within the top ten for each of those months when compared to the full 40-year dataset (ranked 8th and 6th, respectively).

  • SSTs in 2023 were consistent with the long-term trend of increasingly warm conditions driven primarily by anthropogenic climate change.

Annual Average 2023 SST Compared to Historical Conditions

When we compare the annual average SST in the Gulf of Maine for 2023 (52.67°F) to other years, we can see it narrowly beat out 2012 as the second warmest year on record. When we look at the deviation from the long-term average SST (i.e., the annual SST anomaly), the last decade stands out for its exceptional warmth (Figure 6). 2023 extends a pattern that began in 2010 of sustained above-average temperatures. With the exception of 2019 (ranked 13th), every one of the last ten years remains in the top 10 warmest years on record.

Figure 6: A ranking of the top 10 annual SST values [bars] and those years’ respective SST anomalies [x-axis].

Comparison to Global Temperatures

It was not just the Gulf of Maine that experienced exceptional warmth in 2023, however. Globally, 2023 was the 1rd warmest year for sea surface temperatures, and 6th-warmest year for combined land & ocean temperatures.. Figure 7 shows annual average SST anomalies for oceans all over the world in 2023. While much of the Southern Ocean and expanses of the southeastern Pacific were anomalously cool (a general feature of La Niña conditions), most of the world’s oceans experienced unusually warm temperatures in 2023. This is particularly true for northern midlatitudes, especially the northwestern Pacific (off Japan) and along the Eastern Seaboard of the United States—a region that includes the Gulf of Maine.

Figure 7: Map of annual average sea surface temperature anomalies for the world’s oceans in 2022.

Daily Sea Surface Temperatures

The annual cycle of SST in the Gulf of Maine exhibits a pattern common to regions in the Northern Hemisphere, with the lowest temperatures observed in March and the highest values observed in August (Figure 8). Daily SST anomalies in 2023 never fell below +-0.67 °F compared to the long-term (1982-2011) average and reached as high as high as 6 °F above the long-term average. The largest temperature anomalies were observed during November, which also turned out to be the warmest November on record (as highlighted in our Fall 2022 update). Figure 8 also illustrates that, in 2023, 353 days experienced SST anomalies that exceeded the threshold for being considered marine heatwaves, or MHW (more on MHWs below, including Figure 10)

A line chart shows the average sea surface temperature for each day of the year, and overlaid against it are notably smoother black lines representing the long-term mean, 10th percentile, and 90th percentile temperatures for those same days of the year. The area between the mean and the observed temperature is filled in with a solid color to indicate whether that day meets marine heatwave criteris. The observed temperatures for 2022 are above the 90th percentile for nearly the entire year, with the exception of two periods less than a week long in June and October.

Figure 8: A timeseries of marine heatwave (MHW) conditions in the Gulf of Maine extending from January 1 through December 301 2022. Black lines representing the long-term (i.e., 1982 – 2011) average SST, the 10th percentile (i.e., cold spell threshold), and 90th percentile (i.e., heatwave threshold) for a given day in the Gulf of Maine are labeled to indicate climatological reference points. A solid line (red for marine heatwave or blue for a non-heatwave day) indicate the observed SST for each day; red and blue shading illustrates whether each day is considered part of a MHW event.

Daily SST values in 2023 were some of the highest ever recorded in the Gulf of Maine.Record daily high SSTs were set for more than half of all days in November and December, with new records set in all months but July (Figure 9). The most daily records in a single month occurred in November, which experienced record high SSTs for 24 of the month’s 30 days. In total, record high daily temperatures were reached on 169 days in 2023— stated another way: nearly 46% of the year experienced record high SSTs.

Figure 9: An illustration of the percentage of days during each month in 2022 when a record-high temperature was observed in the Gulf of Maine (e.g., 80% — 24 days — of November were new record-setting high temperatures).

More Persistent, Intense Heatwaves in Recent Years

A marine heatwave (MHW) is defined as a period when there are five or more consecutive days when the observed SST is greater than the 90th percentile of the long-term average for that day. Gaps of 2 days or less in this threshold do not constitute a break in the MHW event. Figure 8 illustrates that the Gulf of Maine met the criteria for experiencing a MHW for 231 days in 2023 (or 97% of the year). Superimposing MHW status (black line) over the full timeseries of daily SST anomalies (blue/white/red shading) (Figure 8) reveals that the frequency, duration, and intensity of MHWs has increased in the past decade.

In a world without human-caused climate change, we would expect, positive (warm) and negative (cool) SST anomalies to more or less balance out over the span of several years, as various patterns of natural climate variability alternate having a dominant influence on Earth’s climate (e.g., La Niña vs El Niño). What is being observed in the Gulf of Maine (and elsewhere around the world), however, is a loss of that balance: larger fractions of recent years are experiencing above average temperatures and cold spells are becoming vanishingly rare.

A figure displays the temperatures for each day of year as a colored stripe, organized with a row for each day such that the day of year aligns vertically. The lower two-thirds has a roughly equal balance between colors for warm (red) and cold (blue) temperature anomalies. The top third of the image is almost completely red as temperature anomalies rarely fall below the long-term average. Black dots are overlaid onto days that meet the criteria for a heatwave, they are rare in the lower section and common in the red section.

Figure 10: Heat map of daily SST anomalies from the beginning of 1982 through the end of 2022. Not only do more large warm anomalies (darker reds) appear more frequently in recent years, but the frequency and duration of marine heatwave events (black lines) in the Gulf of Maine has become more pronounced in the past decade.

Forthcoming Methodological Changes to GMRI SST Updates

These seasonal and annual temperature updates rely on a climatological reference period—referred to as a “normal” in the climate science community—against which we calculate anomalies. Starting with our next update, we will be adjusting this “normal” period in our calculations. To date, we have used 1982-2011 as the 30-year climatological reference period, and going forward, we will be using 1991-2020 as our climatological reference period.

This update is standard practice: as time progresses, more data are collected, and new 30-year “normals” can be calculated every decade. However, since the period of 1991-2020 was warmer than 1982-2011, we can expect warm anomalies to be lower than those we report in this and prior updates. To be clear, this result will be merely an artifact of the shifting reference period, not an indication that warming in the Gulf of Maine has decreased.

In addition, we will be modifying how we characterize and communicate about marine heatwaves. The concept of a marine heatwave emerged as a way to communicate a rare phenomenon where temperatures were much higher than historical conditions for a defined duration of time. As temperatures increase globally, SST observations that were once rare have become more common. Understanding this pattern, scientists now are considering new ways to define and communicate about marine heatwaves. GMRI scientists are part of this evolving discussion in the international scientific community, and we will incorporate alternative approaches for characterizing marine heatwaves in future updates. We are producing a technical report to explain these changes in greater detail. Its availability will be announced on our website and linked in our 2023 SST updates.


A Note on Data Sources

The figures in this report are created using remotely-sensed satellite data as part of publicly funded research efforts. Satellite SST data was obtained from NOAA’s National Center for Environmental Information (NCEI), with all maps and figures displaying NOAA’s Optimum Interpolation Sea Surface Temperature Data.

  • NOAA High Resolution SST data provided by the NOAA/OAR/ESRL PSL, Boulder, Colorado, USA, from their Web site at https://psl.noaa.gov/data/gridded/data.noaa.oisst.v2.highres.html.

Recommended Citation:

Gulf of Maine Research Institute. 2023. Gulf of Maine Warming Update: 2023. https://gmri.org/stories/warming-22


OLD ASSETS:

Temperature Anomaly Horizons

One way to think about the severity of these changes is to think about temperature horizons. A temperature horizon captures how long temperatures remain above certain thresholds. Each threshold is designated its own temperature, and in this way we can see how long within a year temperatures remained: 1 to 2 or as much as 4\(^oC\) above normal.

Horizon plot of all years and their temperature anomaly horizons

Figure 11: Temperature anomaly horizons for all years of available satellite data (1982-2023). Horizons display how long temperatures were above certain thresholds (horizon width) and are colored by the strength of these events.

Comparing the Gulf’s Two Hottest Years

If we pull the horizons of our two hottest years on record it makes it easier to contrast where either one experienced acute high temperature events, and where there were sustained periods of above average temperatures.

Early into 2023 it was apparent that the year was on-par with the previous title-holder for warmest year on record.

Single year horizon plots comparing 2021 and 2022

Figure 12: Two horizon plots compare the timing and duration of elevated temperatures in the two warmest years on record, 2021 and now 2022. The width of a color horizon indicates the duration that temperatures were above that threshold.

The Balance of Hot and Cold

Another way to visualize the climate transition that we are observing is by looking at the fraction of each year spent in different temperature ranges. Under a steady climate we would expect over the long-term to spend similar amounts of the year experiencing relatively warm & cold temperatures. These periods would balance themselves out and we would on-average have experienced something similar to the long-term climate.

What we have been seeing in the Gulf of Maine recently has lost that sense of balance. Larger fractions of the year are shared by above average temperatures & cold spell events are becoming vanishingly rare.

Streamplot tracking fraction of each year spent in varying degrees of temperature anomalies

Figure 13: A streamplot shows the fraction of each year on record that temperature anomalies fell within ranges further from the climate reference period average. Since 2010 the Gulf of Maine’s temperatures shifted rapidly outside of this expected range and a large fraction of the year is now 2 degrees or more above that average (red).
 

A work by Adam A. Kemberling

Akemberling@gmri.org